Chirality-switchable circularly polarized luminescence in solution based on the solvent-dependent helix inversion of poly(quinoxaline-2,3-diyl)s

Article abstract of DOI:10.1039/c4cc03944k

Poly(quinoxaline-2,3-diyl)s containing (S)-2-methylbutoxy side chains were found to exhibit blue circularly polarized luminescence (CPL). The handedness of the CPL could be switched by a solvent-dependent helix inversion of the polymer backbone between chloroform (M-helical structure) and 1,1,1-trichloroethane (P-helical structure).

Full text of DOI:10.1039/c4cc03944k

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Chirality-switchable Circularly Polarized
Luminescence in Solution Based on
the Solvent-dependent Helix Inversion of
Poly(quinoxaline-2,3-diyl)s
Yuuya Nagata,a Tsuyoshi Nishikawa,a and Michinori Suginome*ab
enantiomeric excess. Herein, we report the dependence of the CPL
emission of poly(quinoxaline-2,3-diyl)s on the helical sense of the
polymer backbone, which can be controlled by solvent effects.
Poly(quinoxaline-2,3-diyl)s containing
(S)-2-methylbutoxy
side chains were found to exhibit blue circularly polarized
luminescence (CPL). The handedness of the CPL could be
switched by a solvent-dependent helix inversion of the
Firstly, we measured the UV-vis absorption of 200mers 1(200),
2(200), and 3(200), as well as of monomeric model compound 4 in
CHCl3 (Figure 1A). All compounds showed similar bimodal
absorption peaks arising from the -* transition of the quinoxaline
ring. Photoluminescence (PL) spectra of 1–4 showed that these
compounds also exhibit blue fluorescence emission (Figure 1B). The
PL intensity of polymer 3(200) with alkoxy side chains was
significantly stronger than that of 1(200) and 2(200), where the
chiral side chains are connected via a carbon atom. Absolute
fluorescence quantum yields for 1(200) and 2(200) in CHCl3
solution were found to be 0.3%, and 0.7% for 3(200). Even though
these fluorescence quantum yields were low, the fluorescence
emission could still be observed visually (Figure 1D), due to the
large molar absorption coefficients (log ε = 3.97 at 365.0 nm per
monomer unit). The significantly stronger fluorescence of polymer
3(200) relative to 1(200) and 2(200) may be explained by the
increased electron donating ability of the alkoxy groups, which are
directly attached to the electron-deficient quinoxaline ring. Polymer
3(200) exhibited a broader emission peak compared to model
compound 4, probably due to excitonic interactions between
quinoxaline rings. Circular dichroism (CD) spectra of polymers
1(200), 2(200), and 3(200) clearly showed a Cotton effect, reflecting
their previously reported helical main chain (Figure 1C). On the
other hand, the CD spectrum of model compound 4 did not show any
peaks, corroborating that the CD peaks of the polymers 1-3 originate
exclusively from the helical arrangement of the quinoxaline rings.
polymer backbone between chloroform (
M-helical structure)
and 1,1,1-trichloroethane ( -helical structure).
P
Circularly polarized luminescence (CPL)1 is defined as the
differential emission of right- and or left-handed circularly polarized
light. CPL has attracted considerable interest in the past, not only
due to its capacity to elucidate chiral structures in the excited state,2
but also because of potential applications in chemical sensors,3
biological probes,4 and three-dimensional displays.5 Much effort has
been devoted to the development of CPL materials based on small
chiral molecules6 and helical macromolecules7 in aggregates or the
solid state. Examples of materials exhibiting CPL in a non-
aggregated state, on the other hand, still remain limited, despite the
expectation of the development of CPL materials operating in
solution. To date, chiral lanthanide complexes,8 sterically strained π-
conjugated
compounds,9
and
supramolecules
exhibiting
intramolecular excimer emission10 have been described as examples
of non-aggregated CPL materials. Recent research interest has
focused on the switch of the CPL handedness, for example by using
achiral external stimuli11 such as solvent,12 non-polarized light,13 or
an achiral matrix.14 However, only few examples exhibit an
inversion of CPL chirality in aggregates or the solid state, and no
example of CPL inversion in dilute solution has been reported yet.
Recently, we reported the solvent-dependent switch of helical
chirality in poly(quinoxaline-2,3-diyl) containing chiral side chains
using either chloroform (CHCl3, >99% se, P-helix) or 1,1,2-
trichloroethane (1,1,2-TCE, >99% se, M-helix) as solvents.15 We
also demonstrated that poly(quinoxaline-2,3-diyl)s bearing
diarylphosphino groups can serve as highly effective chiral
supporting ligands for transition metal catalysts in various
asymmetric reactions.16 The chirality of these catalysts can be
inverted in a controlled fashion by the choice of solvent, resulting in
the formation either R- or S-enantiomers with high levels of
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solvents (Figure 2B).
Figure 2. (A) Correlation between the degree of polymerization n and the
screw-sense excess (se) of the polymers 3(n) (n = 30, 40, 60, 80, 100, 150,
and 200) in CHCl3 and 1,1,1-TCE. (B) CD spectra of 3(200) in CHCl3 and
1,1,1-TCE.
Figure 1. (A) UV-absorption, (B) PL, and (C) CD spectra of polymers
1(200) (4.9 × 10–5 M), 2(200) (7.3 × 10–5 M), 3(200) (6.5 × 10–5 M), and
monomeric model compound 4 (1.0 × 10–5 M) in CHCl3. (D) Photograph of
1-4 in CHCl3 (1.0 × 10–5 M) under UV light irradiation (λex = 365 nm).
Finally, we measured the CPL of 3(200) in dilute solutions of
several organic solvents including CHCl3 and 1,1,1-TCE (Figure 3,
for spectra in other solvents, see SI). Indeed, CPL signals of 3(200)
could be observed even in dilution, indicating that the signals
observed stem from the helical conformation of the quinoxaline
rings. We also confirmed that the concentration of the solutions did
not affect the UV, CD, PL, and CPL spectra in the range of 1.3 × 10–
We then became interested in the solvent-dependent helix
inversion of polymer 3. As discussed in our previous paper,15b
polymeric 40mer 3(40) adopted an M-helical conformation in CHCl3
and a P-helical conformation in 1,1,2-TCE. Unfortunately, polymers
of 3 with higher molecular weight (100mer and 200mer) were
insoluble in 1,1,2-TCE. Experimenting with different solvents, we
found that 1,1,1-trichloroethane (1,1,1-TCE) was both able to cause
the solvent-dependent helix inversion of polymer 3 and dissolve the
100mer and 200mer. In order to determine the gained energy
difference per monomer unit (ΔGh) and the dissymmetry factor gabs
for the purely single-handed helical structure (gmax) in CHCl3 and
1,1,1-TCE, we prepared seven polymers with (S)-2-methylbutoxy
side chains and different degrees of polymerization (30–200mer).
All polymers exhibited M-helical structures in CHCl3 and P-helical
structures in 1,1,1-TCE. In accordance with Green’s theory,17 the
screw-sense excesses (se) increased in a non-linear fashion in
parallel to increased degrees of polymerization (Figure 2A). The gmax
values at 366.0 nm were found to be –2.29 × 10–3 (CHCl3) and 1.55
× 10–3 (1,1,1-TCE), both of which are comparable to previously
reported polymer systems.15b Values for ΔGh were found to be 0.07
kJ/mol (CHCl3) and 0.14 kJ/mol (1,1,1-TCE).18 These results
indicate that 3(200) adopted at room temperature exclusively (>99%
se) P- or M-helical conformations in CHCl3 or 1,1,1-TCE,
respectively. The CD spectra of 3(200) in CHCl3 and 1,1,1-TCE
were almost perfect mirror images of each other, indicating almost
absolute solvent-dependent helix inversion between these two
5
to 3.2× 10–4 M (see SI). Again, these CPL signals were almost
identical mirror images of each other. The CPL dissymmetry factor
glum is defined as glum = 2(IL – IR)/(IL + IR), where IL and IR are the
fluorescence intensities of the right- and left-handed circularly
polarized light, respectively. Maxima for |glum| were found at 2.3 ×
10–4 (CHCl3) and 4.1 × 10–4 (1,1,1-TCE), which are comparable with
previously reported non-switchable CPL materials in solutions
(typical |glum| = 10–5–10–2).9d The dissymmetry factors of absorbance
(gabs) can be obtained from CD and UV spectra and were found to be
10 and 3.8 times larger than the corresponding glum values in CHCl3
and 1,1,1-TCE, respectively. It should be noted that polymers 1(200)
and 2(200) also showed CPL peaks in CHCl3, although model
compound 4 did not exhibit any CPL in CHCl3 or 1,1,1-TCE (see SI),
further confirming that the CPL emission of 3(200) originated solely
from the helical alignment of the quinoxaline rings.
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†Electronic Supplementary Information (ESI) available:
Experimental procedures and characterization data for new
compounds. See DOI: 10.1039/b000000x/
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In summary, we have investigated the photoluminescence
properties of single-handed helically chiral poly(quinoxaline-2,3-
diyl)s containing chiral side chains in CHCl3 and 1,1,1-TCE. A
polymer containing (S)-2-methylbutoxy side chains unambiguously
showed a solvent-dependent helix inversion between a purely P-
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TCE. The polymer moreover exhibited solvent-dependent chirality
inversion of the CPL in dilute solution. The CPL spectra in CHCl3
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non-switchable CPL materials in solution. Further studies on the
development of other fluorescent and chirality-switchable helical
poly(quinoxaline-2,3-diyl)s as a new class of CPL materials are
currently in progress in our laboratory.
This work is supported by the Japan Science and Technology
Corporation (CREST, “Development of High-performance
Nanostructures for Process Integration” Area) and in part by a Grant-
in-Aid for Scientific Research from Ministry of Education, Culture,
Sports, Science, and Technology, Japan. The authors are grateful to
Prof. Yoshiki Chujo, Prof. Yasuhiro Morisaki, and Mr. Masayuki
Gon (Department of Polymer Chemistry, Graduate School of
Engineering, Kyoto University) for CPL measurements.
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Notes and references
a Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan.
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E-mail: suginome@sbchem.kyoto-u.ac.jp
b JST, CREST (Creation of Next-Generation Nanosystems through
Process Integration), Kyoto University, Katsura, Nishikyo-ku, Kyoto
615-8510, Japan.
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18 In our previous report,15b although ΔGh of polymer
3 in CHCl3 was
determined as 0.10 kJ/mol by measurement of only 3 types of polymers
(40, 100, and 200mers), the newly determined value 0.07 kJ/mol was
more plausible because it was determined by the measurement of 7 types
of polymer
3 (30 to 200mers).
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